1. Field of the Invention
This invention relates to manholes for access to fill pipes from underground storage tanks, and more particularly to a manhole providing a liquidtight construction for containing spills encountered during the filling process.
2. Description of the Prior Art
Because of the growing number of stringent state and local regulations related to leaks in underground fuel storage tanks, a new tank installation system has been developed which will protect ground water from spills, overcome the disadvantages of concrete vaults, and permit early detection of leaks. This tank installation system requires the deliberate maintenance of the tank in a wet hole, i.e., in a hole in the ground in which the tank is surrounded by a pool of water. Because of the difference in specific gravity of gasoline or other petroleum fuel products in water, the development of a leak in the tank then results in ground water from the wet hole flowing into the tank rather than fuel or other petroleum product in the tank flowing out. When a leak occurred in prior tank installations located in a dry hole, the product level in the tank dropped, indicating a leak. However, with a "wet hole" tank installation, water flows in through the leak opening when a leak occurs, and the product level in the tank begins to rise, indicating a leak.
To install a "wet hole" type tank system, the tank hole is lined with a heavy duty impervious membrane, preferably a membrane with a high degree of elasticity. The membrane should be resistant to all forms of hydrocarbon, and such other fuels as methanol. This liner is placed in the bottom of the hole on a pallet and unfolded to cover the sides of the hole to grade level. The tank is balanced through the use of the tie-down straps attached to a concrete pad poured over the liner in the bottom of the hole, or through the use of side ballasts. The tank excavation is then back-filled in the normal manner, and the excavation is flooded with water inside the liner. Finally, the driveway slab is poured over the top of the excavation. The tank is thus completely submerged in the water which is retained within the excavation liner. In the event of a catastrophic spill, a tank rupture for example, the spill product is retained within the liner. In the event of a small leak, water enters the tank, causing the liquid level of the product to rise.
An observation well or manhole should be installed outside the liner at one end of the tank. Prior manhole constructions have generally been unsuitable for such installations because they did not maintain an adequate watertight seal to prevent the water in the liner surrounding the tank from entering the manhole or to prevent spilled fuel from draining into the containment area. Such a manhole should have sufficient flexibility to permit any misalignments of the riser pipe leading up from the tank and to permit the riser to move due to ground shifts when moisture in the soil freezes.
Liquid tight manholes also have applications in which they are installed in underground tank installations which are not of the "wet hole" type. These liquidtight manholes are advantageous as a containment measure in the event that fuel or other product is spilled during the filling process. A liquid tight containment manhole reduces the pollution of ground water and product spillage which may occur during the filling of the underground tank by capturing the overspill and holding it in the containment manhole. The contained product may then be removed from the manhole, or may be drained directly into the underground tank.
Examples of such containment manholes are shown in U.S. Pat. No. 3,633,219, issued to Byrd; patent No. 4,278,115, issued to Briles et al.; and patent No. 4,527,708, issued to Bundas et al. The Briles et al. and Bundas et al. patents show containment manholes each having a flexible seal around the riser pipe at the location at which the riser pipe enters the manhole. The Briles et al. patent also discloses a drain within the manhole through which the spilled product may be removed from the manhole. While Briles et al. disclose draining the product to a separate holding tank, the Byrd patent discloses connecting an exterior drain to the riser pipe so that spilled material may be drained directly into the riser. While these designs provide certain advantages, there are various difficulties resulting from the design of these manholes.
One problem relates to the drainage of the spilled product in the manhole from the manhole into the underground tank. Prior art designs have typically used an external hose connecting the drain in the manhole to a fitting on the riser. A flexible hose has typically been used. However, this hose is susceptible to external damage and internal blockage. Blockage could occur due to debris contained in the manhole which would be drained into the drain hose and block the drain. Damage could result during installation, or due to frost heave or settling, possibly resulting in a leak to the hose or resulting in the hose breaking or becoming disconnected from the riser. Because the manhole is externally fitted and back-filled, it is extremely difficult to remove the manhole to repair the drain hose without excavation of the entire site.
Another problem with the prior art containment manholes related to the use of the elastomeric seal to achieve integrity between the riser pipe and the opening in the manhole through which the riser extended. When the manhole moved axially due to frost heave or settling, the movement could introduce debris into the seal gland and cause damage to the seal or to the surface which the seal must act upon. If either the seal or the surface were damaged, the manhole would no longer contain liquid, since liquid could leak through the damaged seal around the riser pipe.
Another problem also related to the types of seals employed in the prior art manholes. The seals used in the prior art manholes required rather precise glands to hold the seal in a particular relationship to the seal surface. This requirement did not allow for radial movement of the manhole. The manhole might move radially due to tank settling, pavement settling, thermal expansion of the surrounding pavement, or seismic shock. Such radial movement could cause the seal gland to bind and damage the seal or the surface against which the seal acts. Furthermore, radial movement of the manhole could result in the opening in the manhole binding against the riser pipe, resulting in severe stresses transmitted through the riser pipe which could cause damage to the pipe or to the underground tank.
Prior art manholes of both the containment type and conventional non-containment type also had problems in the design of the lid used to close the manhole. Prior art lids typically have sealingly engaged the rim to prevent surface water on top of the lid from draining into the manhole. To reduce the reliance upon this sealing engagement, some prior art lids have been provided with radially extending drainage channels to channel the water from the top of the lid into a trough on the rim of the manhole surrounding the lid. However, water could accumulate in this trough and could freeze in cold weather, making it difficult to remove the lid. This ice formation acted to bond the lid to the rim, and required that the ice be chipped away or otherwise removed before the lid could be pried off the manhole rim.
Another problem with prior art manhole lids resulted from the interrelationship between the lid and the rim. Typical prior art lids were only slightly larger than the top opening of the manhole. Thus, if the lid was not placed exactly over the opening of the manhole, sealing engagement would not be achieved. Furthermore, it would be possible for the lid and rim to close in such a manner that would allow the lid to lift or flip if a load was applied to a portion of the lid. This dislocation of the lid could also create an opportunity for personal injury if a pedestrian were to step on the displaced manhole and slip into the opened manhole. Additionally, it was possible for the lid to lift up and damage the vehicle which might pass over the manhole. To prevent these occurrences, manholes of the prior art were typically formed of metal and made as heavy as possible to reduce the possibility of flipping. However, the resulting manhole lid was difficult to remove and expensive to fabricate.